Communities of mycorrhizal fungi in different trophic types of Asiatic Pyrola japonica sensu lato (Ericaceae)

Green, variegated, and albino Cremastra variabilis provide insight into mycoheterotrophic evolution associated with wood-decaying fungi

With approximately 31,000 species, orchids begin life as mycoheterotrophs, relying on fungi to meet their carbon demands. Notably, some green orchids retain the ability to acquire carbon through fungal associations (partial mycoheterotrophy) and occasionally produce albino or, more rarely, variegated phenotypes. A linear relationship has been observed between leaf chlorophyll content and dependence on fungal-derived carbon, particularly in orchids associated with ectomycorrhizal (ECM) fungi, but whether such plasticity is similarly robust among orchids associated with non-ECM fungi remains underexplored. Here, we focused on the green, variegated, and albino forms of Cremastra variabilis, which likely lack ECM associations, to investigate (i) whether the degree of mycoheterotrophy, indicated by 13C enrichment, correlates with chlorophyll content, and (ii) whether nutritional shifts align with changes in plant structure and mycorrhizal communities. Our results show that rhizoctonia fungi were dominant in green individuals with high chlorophyll levels and lacking coralloid rhizomes, whereas albino and most variegated individuals possessing coralloid rhizomes primarily associate with Psathyrellaceae fungi. Chlorophyll content and carbon stable isotope abundances were negatively correlated, indicating a gradient of increasing mycoheterotrophy from green to albino forms in individuals with coralloid rhizomes. In conclusion, C. variabilis maintains a flexible balance between photosynthesis and mycoheterotrophy, likely shaped by its subterranean morphology and fungal associations, with wood-decaying Psathyrellaceae fungi providing greater support for mycoheterotrophic nutrition than rhizoctonia fungi.

Rhizosphere Bacterial Communities Alter in Process to Mycorrhizal Developments of a Mixotrophic Pyrola japonica

Rhizosphere bacteria work in synergy with mycorrhizal fungi to promote plant growth. The community structure of rhizosphere bacteria may be influenced by continuous changes in fungal associations with host plants. Asiatic herbaceous plant Pyrola japonica (Ericaceae) forms arbutoid mycorrhizas without fungal mantles, with its mycorrhizal development being visually distinguishable at the cellular level. This study aimed to investigate roles of rhizosphere bacteria and their community shifts along with mycorrhizal developments. We examined bacterial communities at three different developmental stages of mycorrhizal roots-limited, full, and digested-via a partial 16S rRNA amplicon sequencing. Both α- and β-diversities in the full condition were significantly lower than those in the limited and digested conditions. Significant clusters of bacterial compositions were found among all treatments. In terms of ecological processes of community assembly, communities in limited conditions and bulk soil were influenced by both deterministic and stochastic processes, whereas those in full and digested conditions were regulated only by stochastic ways. Furthermore, the order Rhizobiales and Actinomycetales known as mycorrhizal helper bacteria were characterized in the full and digested conditions through phylogenetic analysis and detection of indicator taxa. These results suggest that mycorrhizal fungi may play ecologically important roles not only as temporal drivers initiating the formation rhizosphere bacterial communities but also as key founders exerting continuous influences to establish priority effects. Moreover, the rhizosphere bacterial community remains after mycorrhizal degeneration and their historical continuity may contribute to maintaining plant-mycorrhizal fungi-bacterial associations.

Japonolirion osense, a close relative of the mycoheterotrophic genus Petrosavia, exhibits complete autotrophic capabilities

The plant kingdom exhibits a diversity of nutritional strategies, extending beyond complete autotrophy. In addition to full mycoheterotrophs and holoparasites, it is now recognized that a greater number of green plants than previously assumed use partly of fungal carbon. These are termed partial mycoheterotrophs or mixotrophs. Notably, some species exhibit a dependency on fungi exclusively during early ontogenetic stages, referred to as initial mycoheterotrophy. Japonolirion osense, a rare plant thriving in serpentinite soils, emerges as a potential candidate for initial mycoheterotrophy or mixotrophy. Several factors support this hypothesis, including its diminutive sizes of shoot and and seeds, the establishment of Paris-type arbuscular mycorrhizal associations, its placement within the Petrosaviales-largely composed of fully mycoheterotrophic species-and its ability to face the challenging conditions of its environment. To explore these possibilities, our study adopts a multidisciplinary approach, encompassing stable isotope abundance analyses, in vitro experiments, anatomical analyses, and comparative plastome analyses. Our study aims to (1) determine whether J. osense relies on fungal carbon during germination, indicating initial mycoheterotrophy, (2) determine if it employs a dual carbon acquisition strategy as an adult, and (3) investigate potential genomic reductions in photosynthetic capabilities. Contrary to expectations, our comprehensive findings strongly indicate that J. osense maintains complete autotrophy throughout its life cycle. This underscores the contrasting nutritional strategies evolved by species within the Petrosaviales.

Effects of fungicide treatments on mycorrhizal communities and carbon acquisition in the mixotrophic Pyrola japonica (Ericaceae)

Pyrola japonica, a member of the family Ericaceae, is a mixotroph that grows on forest floors and obtains carbon (C) from both its photosynthesis and its mycorrhizal fungi. Its mycorrhizal community is dominated by Russulaceae. However, the mechanism of its C acquisition and its flexibility are not well understood. Our aim was to assess the impact of disturbance of the mycorrhizal fungal communities on C acquisition by P. japonica. We repeatedly applied a fungicide (Benomyl) to soils around P. japonica plants in a broad-leaved forest of central Japan, in order to disturb fungal associates near roots. After fungicide treatment, P. japonica roots were collected and subjected to barcoding by next-generation sequencing, focusing on the ITS2 region. The rate of mycorrhizal formation and α-diversity did not significantly change upon fungicide treatments. Irrespective of the treatments, Russulaceae represented more than 80% of the taxa. Leaves and seeds of the plants were analysed for 13C stable isotope ratios that reflect fungal C gain. Leaf and seed δ13C values with the fungicide treatment were significantly lower than those with the other treatments. Thus the fungicide did not affect mycorrhizal communities in the roots, but disturbed mycorrhizal fungal pathways via extraradical hyphae, and resulted in a more photosynthetic behaviour of P. japonica for leaves and seeds.

Mycoheterotrophy in the wood-wide web

The prevalence and potential functions of common mycorrhizal networks, or the 'wood-wide web', resulting from the simultaneous interaction of mycorrhizal fungi and roots of different neighbouring plants have been increasingly capturing the interest of science and society, sometimes leading to hyperbole and misinterpretation. Several recent reviews conclude that popular claims regarding the widespread nature of these networks in forests and their role in the transfer of resources and information between plants lack evidence. Here we argue that mycoheterotrophic plants associated with ectomycorrhizal or arbuscular mycorrhizal fungi require resource transfer through common mycorrhizal networks and thus are natural evidence for the occurrence and function of these networks, offering a largely overlooked window into this methodologically challenging underground phenomenon. The wide evolutionary and geographic distribution of mycoheterotrophs and their interactions with a broad phylogenetic range of mycorrhizal fungi indicate that common mycorrhizal networks are prevalent, particularly in forests, and result in net carbon transfer among diverse plants through shared mycorrhizal fungi. On the basis of the available scientific evidence, we propose a continuum of carbon transfer options within common mycorrhizal networks, and we discuss how knowledge on the biology of mycoheterotrophic plants can be instrumental for the study of mycorrhizal-mediated transfers between plants.

Partial and full mycoheterotrophy in green and albino phenotypes of the slipper orchid Cypripedium debile

Most green orchids form mycorrhizal associations with rhizoctonia fungi, a polyphyletic group including Serendipitaceae, Ceratobasidiaceae, and Tulasnellaceae. Although accumulating evidence indicated that partial mycoheterotrophy occurs in such so-called rhizoctonia-associated orchids, it remains unclear how much nutrition rhizoctonia-associated orchids obtain via mycoheterotrophic relationships. We investigated the physiological ecology of green and albino individuals of a rhizoctonia-associated orchid Cypripedium debile, by using molecular barcoding of the mycobionts and stable isotope (¹³C and ¹⁵ N) analysis. Molecular barcoding of the mycobionts indicated that the green and albino individuals harbored Tulasnella spp., which formed a clade with the previously reported C. debile mycobionts. In addition, stable isotope analysis showed that both phenotypes were significantly enriched in ¹³C but not in ¹⁵ N. Therefore, green and albino individuals were recognized as partial and full mycoheterotrophs, respectively. The green variants were estimated to obtain 42.5 ± 8.2% of their C from fungal sources, using the ¹³C enrichment factor of albino individuals as a mycoheterotrophic endpoint. The proportion of fungal-derived C in green C. debile was higher than that reported in other rhizoctonia-associated orchids. The high fungal dependence may facilitate the emergence of albino mutants. Our study provides the first evidence of partial mycoheterotrophy in the subfamily Cypripedioideae. Partial mycoheterotrophy may be more general than previously recognized in the family Orchidaceae.